This invention relates generally to electrochemistry, and more particularly to an electrochemical process achieving simultaneous product synthesis and purification as well as starting material regeneration. The invention thus holds significance across the large and growing scale of electro-syntheses both adopted and being considered for preparative scale reactions.
By way of further background, after having reached a peak in the early 1900's and thereafter having for some time received little attention in the literature and in industry, preparative scale electrochemical synthesis of organic and other chemical products is again growing in technical and commercial import. The literature demonstrates that numerous and diverse products can be formed electrochemically on a laboratory scale. However, more recent industrial attempts and literature pertaining thereto have also shown that there are many problems which must be overcome to render a scaled-up electrochemical synthesis commercially, economically and environmentally practicable.
Several noted limiting factors to the introduction of commercial electro-synthetic reactions are (1) many current large-scale syntheses are founded upon catalytic reactions for which scaling laws have been well established as compared to their electrochemical counterparts, (2) electrochemical systems scale up by area rather than by volume and are thus more costly than secular-type reactions to scale up, (3) electroorganic synthesis in aqueous electrolytes is sensitive to solubility considerations, particularly with respect to substrate solubility, and (4) organic media for electrochemical reactions frequently undergo secondary reactions, which leads to increased production costs from factors such as solvent loss and coating of electrodes. "Industrial Applications of Electroorganic Synthesis," (R. Roberts, R. P. Ouellette and P. N. Cheremisinoff), Ann Arbor Science 1982 pp. 173-174.
In addition to those above, there are many other factors which have prevented the wide scale adoption of preparative electrochemical synthesis. As those in this field will no doubt appreciate and as has also been noted in the literature, a successful electrochemical synthesis in high yield and high efficiency does not necessarily lead to a successful process from a commercial standpoint. Costs stemming from additional operations, raw materials, product and reactant recovery, and pollution and environmental effects are contributing factors of equal or greater importance than cell performance. See ibid., preface p. v. Of these factors, one which has proven particularly critical to the success or failure of a proposed electrochemical preparation is the cost associated with workup of electrolyte mediums to recover the target product. In fact, it is estimated that in most cases more than half of the total process investment is needed for the recovery operation. "Techniques of Chemistry," (N. L. Weinberg and B. V. Tilak) (1982), Vol. V, Part III, p. 279. Consequently, simplifications of product recovery operations most often have a much more pronounced effect in manufacturing cost reduction than do improvements in the cell itself.
In the face of these limiting factors, electrochemical synthesis has to this point offered great promise primarily for isolated, specialized applications in which for one reason or another, usually tied to the specific chemistries involved, the electrochemical process is commercially and environmentally feasible. For example, in a very few number of applications (about 5%), the synthesized product can be easily and inexpensively recovered because it forms a separate phase directly from the electrolyte with no further steps being necessary except possibly a simple cooling or heating cycle. One such example is where the synthesized product is gaseous at temperatures well below the boiling point of the solvent or other volatiles in the electrolyte. For instance, "Techniques of Chemistry," supra. Vol. 5, Part 3, Ch. VII pp. 6-8, describes an electrochemical preparation of fluorinated methanes which evolve from the electrolyte as a gas and are trapped as they escape.
Another situation which allows for a convenient recovery operation occurs where the product is soluble in a second phase such as an extracting solvent. As an example, Monsanto Company of St. Louis, Mo. presently operates an electrochemical process for the commercial production of adiponitrile which functions in this way. Additionally, the product in some cases is formed as a liquid which is insoluble in the electrolyte phase and thus isolation requires a simple phase separation. An example of this phenomenon is found in applicant's own U.S. Pat. No. 4,670,111 assigned to Reilly Industries, Inc. Finally, the product may be formed as a solid which is insoluble in and therefore precipitates from the electrolyte phase. An example of this phenomenon is found in the applicant's own U.S. Pat. No. 4,482,437 assigned to Reilly Industries, Inc. In one specific example, the patent describes an electro-synthesis in which 4-picolylamine sulfate product precipitates from the electrolyte.
While these and other specialized applications have represented important advancements in the field of preparative electrochemistry, equally and perhaps more important advancements are realized by discoveries which are generally applicable over a broad range of electrochemical syntheses. For example, U.S. Pat. No. 4,589,968 to Toomey describes an electrochemical cell which can be used to conduct diverse electrochemical syntheses and thus constitutes an improvement over prior art cells at the time developed and useful only for specific chemistries. Despite this and other advancements which have occurred in the field of electrochemistry, there has for some time existed a need for an improved electrochemical process which provides great advantage not only for isolated chemistries but also for a broad range of electrochemical syntheses. The applicant's invention addresses this need.